The invention relates to a device for destruction-free testing of ferromagnetic component walls such as pipe walls or panel walls with respect to cracks or other elongate defects by ultrasound waves, in particular shear waves, that are excited by a high frequency induction coil in a wall area magnetized in a predetermined direction, that propagate on a path that can be oriented by the induction coil as a sending transducer and that are received at a spacing from the sending transducer by at least one induction coil as a receiving transducer. On different components, in particular, in the exterior area, tests in regard to possible defects are mandatory at predetermined time intervals in order to counteract the risk of failure in the sense of breakage, deformation, or leakage. Walls of pipes as well as walls of containers or the like made from steel are often corrosion-prone. In particular, they can be exposed to stress crack corrosion with typical cracks or crack bundles that penetrate, starting at the surface, into the depth. Likewise, cracks as a result of other causes such as, in particular, overloads are to be considered. In addition, other defects such as corrosion locations are of interest.
Destruction-free testing methods employ, for example, ultrasound methods wherein the excitation of acoustic waves in the wall can be realized from the exterior by means of electromagnetic-acoustic transducers. This so-called EMAT (electromagnetic-acoustic transducer) enables the contactless excitation of acoustic lamb waves, in particular, without coupling medium, in such a wall. In this connection, the excitation of “lamb waves”, as well as of shear waves, that are polarized parallel to the wall surface is much easier and more flexible than by mechanical generation by means of piezo transducers. Such shear waves propagate transverse to the polarization or to the excitation deflection relatively bundled in two opposite directions. These shear waves that must be excited by a suitable frequency for generating a useable excitation with a high frequency induction coil that is geometrically matched particularly with regard to conductor spacings, wherein suitable frequency ranges and conductor spacings vary also with the thickness of the wall, can generate different wave shapes as a function of the depth within the wall. For example, a shear wave of zero order oscillates in the interior of the wall and also at the opposite surface uniformly with the top wall surface while a shear wave of first order forms a node at the center of the wall and at the opposite surface oscillates in the opposite direction. Shear waves of higher order form several nodes within the wall. These wave shapes are suitable in different ways for the detection of defects. Moreover, the oscillation excitation as well as its detection by means of sending or receiving transducers must be detectable as clearly and disruption-free as possible.